1. Field of the Invention
The invention pertains to an ultrasonic detector and more specifically to a light weight, battery-operated, precision instrument for detecting ultrasonic sound or vibrations, which is useful in detecting leaks and malfunctions.
2. Prior Art
It is well known that ultrasonic generators and detectors can be used to locate leaks or defects, e.g. in pipes. Such a system is shown in U.S. Pat. No. 3,978,915 to Harris. In that arrangement, ultrasonic generators are positioned in a chamber through which the pipes pass. At the ends of these pipes, exterior to the chamber, ultrasonic detectors are located. At the point where a leak occurs in the pipe or the pipe wall is thin, the ultrasonic energy will enter the pipe from the chamber and travel to the end of the pipe where the detector is located. The detector will receive an ultrasonic signal at the end of the pipe indicating the existence of the leak or weak spot in the pipe.
Ultrasonic sensors have also been used to detect ultrasonic energy generated by friction within mechanical devices as disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al. The greater the amount of friction, the greater the intensity of the generated ultrasonic energy. Applying a lubricant to the device reduces friction and consequently the intensity of the generated ultrasound drops. Measuring ultrasonic energy thus provides a way to determine when lubrication has reached the friction generating surfaces. Additionally, faulty devices such as bearings generate a higher level of ultrasonic energy than do good bearings, and thus this condition can also be detected. However, conventional means require two people to perform this procedure--one person to apply the lubricant to the device, and one person to operate the ultrasonic detector.
Since ultrasonic energy used for these purposes is generally in the range of 40 kHz, it is too high in frequency to be heard by a human being. Thus, means are typically provided for heterodyning or frequency shifting the detected signal into the audio range, and various schemes are available for doing this.
By locating an ultrasonic generator in a closed chamber, a standing wave pattern with peaks and nodes is established. If a node occurs at the position of a leak or weak spot, no ultrasonic energy will escape and the defect will not be detected. One method of addressing this problem is disclosed in U.S. Pat. No. Re. 33,977 to Goodman, et al. Goodman teaches varying the frequency of the applied ultrasonic energy so that the position of the nodes will shift over time so that a leak at a null or node will be detected. However, resort to this method adds complexity and expense to the testing hardware.
Ultrasonic transducers generally produce a low voltage output in response to received ultrasonic energy. Thus, it is necessary to amplify the detected signal using a high-gain preamplifier before it can be accurately processed. However, if low cost heterodyning and display circuitry are to be used, means must be made available to attenuate the amplified signal to prevent saturating these circuits when high input signals are present. This attenuation also adjusts the sensitivity of the device. For a hand-held unit, the degree of attenuation should be selectable by the user.
For example, U.S. Pat. No. 4,785,695 to Rose et al. discloses an ultrasonic leak detector with a variable resistor attenuator used to adjust the output level of an LED bar graph display. However, this attenuation method does not provide a way to establish fixed reference points to allow for repeatable measurements.
U.S. Pat. No. 5,089,997 to Pecukonis discloses an ultrasonic energy detector with an attenuation network positioned after an initial preamplifier and before the signal processing circuitry, which creates an audible output and an LED bar graph display. The resistors in the Pecukonis attenuation network are designed to provide an exponential relationship between the different levels of attenuation. However, Pecukonis does not heterodyne the detected signals to produce an audible output but rather teaches the benefits of a more complex set of circuits which compress a broad range of ultrasonic frequencies into a narrower audible range. For many applications, the cost and complexity of this type of circuitry is not necessary.
In addition to detecting ultrasonic sound escaping from a leak or defect, a detector using an acoustic transducer must be able to accurately locate the source that sound. To this end, conical sound collectors are used in conjunction with the transducer to increase its directionality as illustrated in U.S. Pat. No. 4,287,581 to Neale, Sr. However, conventional detectors do not utilize collection cones which are also specifically designed to provide additional input signal gain of an amount consistent with the units of measure provided by the detector.
When using ultrasonic energy to detect leaks, it is useful to have a portable ultrasonic sensor which indicates the presence and intensity of ultrasonic energy both visually and audibly. Goodman discloses an ultrasonic sensor which displays intensity of the detected signal on an output meter operable in either linear or logarithmic mode and also provides for audio output through headphones. U.S. Pat. No. 4,987,769 to Peacock et al. discloses an ultrasonic detector which displays the amplitude of the detected ultrasonic signal on a ten-stage logarithmic LED display. However, the detector disclosed in Peacock does not process the detected signal to produce an audible response, nor does it provide for signal attenuation after the initial pre-amplification stage.